On Fossilization

Roy E. Plotnick, Felisa A. Smith, and S. Kathleen Lyons, “The fossil record of the sixth extinction,” Ecology Letters 19 (2016), 546–53

I’d like to go back to the thought experiment that’s at the heart of the stratigraphic Anthropocene: if alien geologists arrived to study the earth in some tens of millions of years, what traces of present-day ecological upheavals might they find? One of the most evocative characteristics of contemporary strata—albeit not the most immediately visible thing about them—would be the last-ever appearances of some species in the fossil record, the indicators of their extinction. But how many species would even leave such a record behind, and how many would simply vanish without a trace?

This paper tackles that question, as regards modern mammal species, by comparing the IUCN Red List of Threatened Species with three databases of fossils. Plotnick, Smith and Lyons make the working assumption that species the Red List calls ‘endangered,’ ‘critically endangered,’ ‘extinct in the wild’ or ‘extinct’ are doomed; and that species in the IUCN’s other categories (‘vulnerable,’ ‘near threatened,’ and ‘least concern’) are going to survive for now. What proportion of the mammals in each of their categories—the doomed and the survivors—are also known to exist in the current fossil record?

At the time of their analysis (the Red List numbers have changed just slightly since then), the figures were as follows:

Hypothetical survivors that do appear in the fossil record: 777

Hypothetical survivors that don’t: 3165

Doomed species that do appear in the fossil record: 68

Doomed species that don’t: 705

That gives them their headline finding: the mammals that are least likely to survive much longer are also the ones least likely to appear anywhere in the fossil record. Just under a fifth of the presumed survivors are also known as fossils, but only 8.8% of the doomed species have a fossil record.

The authors point out that less than 5% of endangered-or-worse rodents are recorded as fossils, compared to 17.3% overall. “Among bats, a dismal three of the 82 threatened species” have left fossils, against 10% overall. Geographic range seems to be a key variable: the smaller your habitat, the more likely you are to go extinct and the less likely you are to leave any trace. Only 6% of doomed island-dwelling species have a fossil record, and species endemic to islands make up a disproportionate number of doomed species generally.

Obviously, there are limits to this analysis. It only looks at mammals, for one thing. If an animal is recorded in a contemporary fossil database, that doesn’t necessarily mean that it will one day be identifiable in hundred-million-year-old rock strata; if it isn’t so recorded, that doesn’t mean its preservation is impossible. Some Red List endangered mammals might yet survive much longer; some merely ‘vulnerable’ animals inevitably won’t. And I’d be inclined to make more of the data on geographic variability than the authors do. Only 6% of doomed island mammals have a fossil record, whereas the proportion is generally higher among doomed mammals on the continents: fine. But it turns out, too, that only 5% of doomed South American mammals and 13% of African ones are known as fossils, compared to 20% or more in Eurasia, North America, and Australia. I’d imagine that that tells us much more about how fossil hunting has been concentrated in the rich world than anything else.

Nonetheless, it’s an interesting analysis. Plotnick, Smith and Lyons don’t have anything to say about the Anthropocene epoch, and the paper isn’t by any means a presented as a contribution to that alien-geologists thought experiment. Instead, their point is that you need to take the differential preservation of fossils into account when comparing the current mass extinction to the great mass extinctions of the distant past. If you don’t, then you might exaggerate the relative size of the modern extinction, by forgetting that the proportion of species that disappear from the fossil record during ancient extinctions is probably lower than the proportion that actually went extinct, since more of the killed-off species would never have left identifiable fossils in the first place. (At least, if the same fossilization pattern holds true for both modern mammals and the tiny ancient shell-forming marine organisms used in assessing past mass extinctions, which very possibly it doesn’t.)

The paper is a fine example, then, of how the alien-geologists, stratigraphic-Anthropocene thought experiment isn’t just scientific whimsy. On the contrary, thinking about the likely stratigraphic traces of the present can be a way to address thoroughly practical issues in conservation and paleontology.

At a distance of a hundred million years, the stratigraphic record wouldn’t be sufficiently fine-grained to distinguish between the time when the dodo and the baiji went extinct, and the episode (beginning around 50,000 years ago) when most large terrestrial mammal species died out as human hunters spread around the world. Those 68 extant, fossilized, and doomed-looking mammals will be overshadowed in the rock record by the late Pleistocene extinctions, even if their disappearance might indeed be discernible many millions of years from now. But the paper does us a service in pointing them out. There they are: future ghosts in future stones.